Semiconductor laser modules can be used as signal transmitters in communication systems. When used in this way, they are often operated in conjunction with external control loops which monitor and correct the wavelength and power of the output. Such loops rely on well-known relations that specify how the wavelength and power of the output can be controlled by varying the operating temperature of, and the drive current supplied to, the laser. Wavelength and power are thus stabilised in many lasers, using modules that are external to the laser module which implement the following types of control loops:
(1) an automatic temperature control (ATC) loop, which accepts a temperature level signal from the laser module and uses it to adjust the operating temperature of the laser to a desired level; and PA1 (2) an automatic power control (APC) loop, which accepts an output power level signal from the laser module and uses it to adjust the input drive current of the laser so that the output power is at a desired level.
Wavelength-error is not directly measured or regulated in many laser cards. Such measurements are omitted on the assumption that the proper functioning of the APC and ATC modules would, by providing control over power and temperature, also provide adequate control over wavelength in accordance with the relations alluded to above.
This assumption is justified when wavelength has to be controlled only to the extent required to prevent mode-hops from occurring. In recent years however, it has become desirable to control wavelengths more closely. More specifically, it has become desirable to upgrade existing laser modules to dense wavelength division multiplexing (DWDM) laser modules in fibre optic transmission systems. This operation of these modules requires reducing the wavelength spacing between channels, which in turn requires a greater degree of wavelength stabilisation than is available on systems that do not directly measure and correct wavelength errors. A control loop for measuring and controlling wavelength errors is necessary to implement support for DWDM. It also remains desirable to simultaneously maintain a control loop for measuring and controlling power.
There are known devices that simultaneously control both power and wavelength. They rely on an APC module that is modified to directly correct any fluctuations in wavelength error, as opposed to output power, by adjusting the drive current into the laser. They control temperature, as is done on pre-DWDM cards, using an ATC module. They minimise output power error by adjusting the temperature target of the ATC module. For examples, see U.S. Pat. No. 4,821,273 by Hori and issued on Apr. 11, 1989, and U.S. Pat. No. 5,042,042, by Hori at al. and issued on Aug. 20, 1991.
The drawback of this approach is that it requires a specialized temperature control circuit which is designed to use a wavelength error signal as its input, and therefore the implementation of wavelength control cannot be inserted into existing systems.
Another problem with this approach, is that it precludes normal operation of an APC module, which is always connected such that its input receives a power level signal, and its output drives a laser current.
A final problem with this approach is that it involves providing the wavelength monitoring apparatus, through the use of an optical splitter, with light from the front facet of the laser. This is problematic because it reduces the power-level of the signal reaching the fibre.
Therefore, existing control loops that simultaneously control wavelength and power, such as those described in U.S. Pat. Nos. 4,821,273 and 5,042,042, cannot be implemented on existing laser cards without affecting the implementation of the already existing APC and/or ATC modules. Since there are many pre-DWDM cards that only contain ATC and APC modules, it is desirable that the new DWDM laser modules be installed without requiring changes to the layout of the existing laser cards, or to the current implementations of the already existing ATC and APC modules. That is, it is desirable for the new wavelength control function to be internalised within the laser module, so as to not disturb the implementation of the already established APC and ATC modules.